The present invention relates to a method of manufacturing a salt of an .alpha.-sulfofatty acid ester by sulfonating a saturated fatty acid ester and thereafter neutralizing the sulfonated ester. To be more precise, it relates to a method of manufacturing a light-colored salt of an .alpha.-sulfofatty acid ester useful as a surface active agent, in high yield.
Salts of .alpha.-sulfofatty acid ester to be obtained by sulfonating saturated fatty acid esters and neutralizing the sulfonated esters thereafter has various advantages such as, they possess not only a satisfactory hard water resistance but also an excellent wettability and are mild to the skin so that they are useful as detergents and wetting agents.
It is known that, when a saturated fatty acid is sulfonated and thereafter neutralized with sodium hydroxide, there can be produced sodium .alpha.-sulfofatty acid ester, disodium salt of .alpha.-sulfofatty acid and monosodium salt of .alpha.-sulfofatty acid as shown in the following: ##EQU1##
The differences of the properties of sodium .alpha.-sulfofatty acid ester, disodium salt of .alpha.-sulfofatty acid and monosodium salt of .alpha.-sulfofatty acid are very great; for instance, as to the water solubility, that of sodium .alpha.-sulfofatty acid ester is about 100 times as high as that of the disodium salt of .alpha.-sulfofatty acid and is about 1,000 times as high as that of the monosodium salt of .alpha.-sulfofatty acid. Also, as to wettability, surface tension, solubilizability, detergency and foamability, sodium .alpha.-sulfofatty acid ester is by far superior to the disodium salt and monosodium salt thereof. This verifies the disodium salt of .alpha.-sulfofatty acid and monosodium salt of .alpha.-sulfofatty acid are unsuitable for use as surface active agents for detergent. Nevertheless, inasmuch as the solubilizability of sodium .alpha.-sulfofatty acid ester is superb, when sodium .alpha.-sulfofatty acid ester is present in an amount of not more than about 80% of the entire surface active agent, even when the remaining 20% or less consists of disodium salt of .alpha.-sulfofatty acid and monosodium salt of .alpha.-sulfofatty acid, said surface active agent can serve as the active ingredient of detergents. However, when the content of sodium .alpha.-sulfofatty acid ester is less than 80%, the detergency, solubilizability and so on decrease precipitously, rendering the surface active agent unfit for use as the active ingredient for detergents. That is, a surface active agent which fails to contain sodium .alpha.-sulfofatty acid ester to the extent of more than 80% is unusable as the active ingredient of detergents.
As to the reaction mechanism for sulfonating saturated fatty acid ester having the formula RCH.sub.2 COOR' with SO.sub.3, it is presumed that this sulfonation reaction progresses through two stages: the first stage where an adduct having the formula ##EQU2## is formed at a high speed from saturated fatty acid ester and SO.sub.3, and the second stage where in a rearrangement reaction of said adduct takes place to form .alpha.-sulfofatty acid ester having the formula ##EQU3## at a low speed. When the speed of forming this .alpha.-sulfofatty acid ester is compared with the speed of forming other anionic surface active agents, for instance, the speed of sulfonation of alkyl benzene, monoolefin or higher alcohol with an appropriate sulfonating agent such as sulfuric anhydride, fuming sulfuric acid or chlorosulfonic acid, the latter speed is very high and the sulfonation reaction is completed instantaneously upon contact with said sulfonating agent, while the former speed is so slow that a certain period of time is required for forming .alpha.-sulfofatty acid ester by rearrangement reaction of SO.sub.3 adduct.
As method of sulfonating fatty acids or derivatives thereof, there have hitherto been known the batch process and the film process: the former is a process comprising effecting the sulfonation reaction by blowing diluted SO.sub.3 gas into fatty acid or derivative thereof and then effecting 10 - 30 minutes' aging by raising the temperature to thereby complete the rearrangement reaction, wherein the aging reaction is indispensable. However, in the case of the batch process reaction, a very long time is required for completion of the entire reaction so that the product tends to be remarkably discolored and become unfit for use in detergents even when a bleaching process is applied thereto. Further, in the case of sulfonation of fatty acid derivatives, especially fatty acid esters thrugh the batch process, inasmuch as SO.sub.3 is employed in excess, that is, the molar ratio of SO.sub.3 to fatty acid ester is almost in the range of 1.0 - 1.5, when the reaction takes a long time, by the catalytic action of excess SO.sub.3, the ester linkage or ##EQU4## is cut owing to the presence of a trifling amount of water, resulting in lowering of the yield of the principal product .alpha.-sulfofatty acid ester while producing .alpha.-sulfofatty acid amounting to more than 30%, so that the sulfonation product obtained through the batch process is unfit for use as the active ingredient for detergents.
Meanwhile, in the case of the method of sulfonating at a high temperature in the range of 95.degree. - 150.degree.C by the use of a film process type reactor such as taught in the specification for British Pat. No. 1145101, improvement of the color tone is admittedly observed as compared with the batch process, yet this method is defective in that, when it is applied to the sulfonation of saturated fatty acid esters and the reaction is effected by applying a high temperature from the outset, unlike the sulfonation of saturated fatty acid, the SO.sub.3 adduct cannot be formed easily (cf. the foregoing description of the reaction mechanism) but the cutting of the ester linkage progresses as the principal reaction, so that not only does the rate of reaction lower to less than 90%, but also the yield of .alpha.-sulfofatty acid ester decreases to less than 80%.
As seen in the foregoing, in the case of sulfonating fatty acid esters, it has hitherto been difficult to obtain a sulfonation product having a satisfactory color and containing more than 80% of .alpha.-sulfofatty acid ester through either the batch process or the film process in the prior art.
Referring to the method of neutralization, the conventional method is to neutralize by adding the sulfonation product of the fatty acid ester to a highly alkaline aqueous solution (with pH value of more than 12). This conventional method, however, has been defective in that, the ester linkage of the .alpha.-sulfofatty acid ester is easily cut by hydrolysis, and when the temperature of neutralization is high, the speed of hydrolysis becomes high to further expedite the cutting of the ester linkage, resulting in a decrease of the yield of the .alpha.-sulfofatty acid ester
The present invention has been accomplished as the result of a series of studies that aimed to overcome the foregoing drawbacks in the conventional methods of manufacturing salts of .alpha.-sulfofatty acid esters, and its object is to provide an improved method capable of producing a light-colored salt of .alpha.-sulfofatty acid esters useful as surface active agents.